GROUPS OF PLANTS. 9 



Pleurococcus. One of the eommonest of the Green Algae 

 as well as one of the simplest is Pleurococcus (Pleurococcus 

 vulgaris) (Fig. 6). It occurs as a green coating, in both winter 

 and summer on the moist bark of trees, moist ground, and 

 stone walls, and is a component of some lichens'. The plant is 

 one-celled, more or less spherical, and at one stage contains a 

 number of chlorophyll grains which finally unite to form a single 

 plate which lies against the wall and is known as a chromato- 

 PHORE. Besides it contains a considerable amount of oil. An 

 allied species (Pleurococcus z'iridis) contains the sugar erythrite. 

 The plant usually reproduces by simple division, that is. one cell 

 or plant divides to form two. The division may continue by the 

 production of another cross wall, so that four cells result. Under 

 favorable conditions, division may take place by the formation 



Fig. 6. Pleurococcus vulgaris. Different stages of division of the cell. After Wille 



of Still another wall at right angles to the other two. In 

 this way two, four and finally eight individuals arise which adhere 

 more or less to one another, thus forming colonies. The number 

 of individuals in a colony depends upon the number of indi- 

 viduals in the colony when division begins and the extent to 

 which division is carried. Thus if there were four cells in a 

 colony to begin with and division took place in three planes, there 

 would be thirty-two cells in the colony at the end of the period. 

 Spirogyra. Another one of the common Green Algae is 

 Spirogyra (Fig. 7), one of the pond-scums, which forms float- 

 ing green masses on ponds and shallow water in the spring. The 

 plant-body consists of a chain of cylindrical cells forming long 

 threads or filaments. The transverse walls are sometimes pecu- 

 liarly thickened. The chromatophores occur in one or more spiral 

 bands (Fig. 7, 77), which extend from one end of the cell to the 



